1. Field of the Invention
The present invention relates to a magnetic toner and an apparatus unit for developing an electrostatic latent image, and an image forming method therefor.
2. Related Background Art
Conventionally, there are many well known electrophotographic methods. Generally, an electric latent image is formed on an image bearing member (photosensitive member) by various means using an optical conductive material, and the latent image is developed by toner to form a toner image and is transferred to a transfer member, such as a paper sheet, as needed, the toner image being fixed to the transfer member by heat or pressure or by heat and pressure to provide a duplicate or a printed material.
Presently, there are various types of devices that use electrophotographic methods, such as copy machines, printers and facsimile machines.
As for printers, for example, LED or LBP printers are a recent trend on the market. Technically, resolutions of printers are increased from conventional 240 or 300 dpi to 400, 600 or 800 dpi. Accordingly, a more detailed development method is required. Also, copy machines tend to have higher functions, so that they are gradually digitized. Since digital copy machines mainly employ a method for forming an electrostatic latent image using a laser beam, the resolutions are increased, and a more detailed development method is required for digital copy machines as well as printers.
Toners having a small particle diameter for which a specific particle distribution is employed have been proposed in Japanese Patent Application Laid-Open Nos. 1-112253, 1-191156, 2-214156, 2-284158, 3-181952 and 4-162048.
When printing is performed using these toners, however, still many toner particles are still scattered around a character line, and an improvement in character line definition or sharpness is required.
Although the character line definition is somewhat improved when a toner having a smaller particle diameter is used, there is a deterioration of flowability of the toner, and a reduction in the density provided for a solid black image is especially noticeable. In addition, as a consequence of the reduction in the diameter of toner particles, fogging tends to occur in a non-image portion.
A preferable magnetic toner having a smaller particle diameter is proposed in Japanese Patent Application Laid-Open No. 1-219756, but requires further improvement for the maintenance of image density and of fogging control.
In addition, in Japanese Patent Application Laid-Open No. 8-101529 (related application: EP-A 0699963) is proposed a magnetic toner containing magnetic fine particles such that a product (.sigma..sub.r .times.H.sub.c) of remanence (.sigma..sub.r Am.sup.2 /kg!) and coercive force (H.sub.c kA/m!) in a magnetic field of 79.58 kA/m (1 k oersted) is 60 to 250kA.sup.2 m/kg!. For the magnetic fine particles that are described in Japanese Patent Application Laid-Open No. 8-101529, a product (.sigma..sub.r .times.H.sub.c) in a magnetic field of 79.58 kA/m (1 k oersted) is 60 to 250kA.sup.2 m/kg!, while a product (.sigma..sub.r .times.H.sub.c) in a magnetic field of 795.8 kA/m (10 k oersted) is approximately 66 to 275kA.sup.2 m/kg!, and magnetic fine particles having the shape of a hexahedron or an octahedron (generally, a sphericity (.psi.) of less than 0.75) are preferably employed. Since the frictional electrification of the magnetic toner is low, -13.0 to -22.0 .mu.c/g, on balance, it is not easy for a magnetic toner that includes a comparatively large number of magnetic toner particles having diameters of 3.17 .mu.m or smaller to provide a high image density and to implement the suppression of fogging occurrences in a non-image portion, and thus, further improvement of the magnetic toner is required.